Comet Hartley 2 is the big observing news for October 2010, but the King of Planets, Jupiter, also will be putting on a prime time show all month with its four dancing Galilean moons. (Jump to Jupiter details here.) Venus and Mars, while technically still visible, are quickly closing in on the Sun in the west. Mercury is diving back towards the Sun in the morning sky, and while Saturn emerges in the morning sky, it won’t be high enough for a decent view until near the end of the month.
Ah, but there’s that comet! A “dirty snowball” that meanders roughly between us and Jupiter as it circles the Sun every six years. Its orbit, according to this wonderful applet at the Jet Propulsion Lab, looks like this. Notice how close it is to our little space station Earth on October 7 – that’s a good thing!
So let’s start the month with Hartley 2. This tiny periodic comet will:
- Come closer to Earth than it ever has before – just 11 million miles;
- Be easily found by binocular users in prime time well away from the Sun – and may even reach naked eye visibility;
- Will be visited by a US space craft November 4, 2010, for some close up images in the hopes of learning more about these fascinating objects.
- Finally, there’s always the possibility that it won’t behave as predicted. It may be dimmer – it may be brighter – even a lot brighter. Probably not – but after dull and dim Comet Holmes shocked everyone by brightening overnight by a factor of about half a million times in October of 2007 – well, comets need watching!
Generally, comets become brightest as they near the Sun – but this also can make them tricky to see. It’s usually a race with two opposing forces conspiring to make life difficult for the observer: As the comet gets closer to the Sun, it gets brighter. But as it gets closer to the Sun it also gets lost in the twilight glow. Such comets are frequently at their best just after sunset, or just before sunrise. This will not be the case with Hartley 2. Comet Hartley 2 will be a prime time comet for Northern Hemisphere observers. Figure on getting your best views of it during the first two weeks of October at a convenient evening hour, such as 8 pm. And circle October 7 as an especially fun time to take a look if the weather gives you a break. What makes October 7 so special? Well, the comet happens to be passing a favorite deep sky object that is also easily visible in binoculars. In fact this object, the famous Double Cluster in Perseus, is relatively easy to find with binoculars and if you can find it, you should be able to spot the comet in the same field of view. Here’s a guide chart I’ve modified some from a Starry Nights Pro screen shot. Notice that the Double Cluster is conveniently placed between two familiar asterisms – the “W” of Cassiopeia and the “Bow” of Perseus. Careful scanning between those two should be enough to reveal it, though the big problem most newcomers have with recognizing deep sky objects with binoculars and small telescopes is they don’t know what they can expect to see. So first here’s what I consider a reasonable simulation of the appearance of the double cluster in typical, hand-held binoculars.
And here’s the guide chart for finding the Double Cluster and Comet Hartley 2 during the first couple of weeks of October.
As the chart above shows, September 30 would be a great time to look for this comet because it will be so near the Alpha star of Cassiopeia – the “W.” Each day it continues in a generally eastward direction. The red circle is a 5-degree field, typical for 10X50 binoculars. (Your binoculars may show a larger field of view.) The comet enters near the top of the circle on October 6, is at the position shown on October 7, and will be exiting the circle at the bottom (eastern side) on October 8. On October 12 it will be about four degrees north of Mirfak. Remember – to find north in the sky just draw a line between a star and Polaris – in this case between Mirfak and Polaris – Comet Hartley should be pretty close to such an imaginary line on October 12.
After that it will continue its eastward path, coming closest to the Earth on October 20. However, after the 12th the moon starts to come into play during prime time and for the next two weeks it will dominate the evening sky, its light making the comet and other faint objects more difficult to see. That’s why the first two weeks of the month are best for this event, but of course the comet can be found later and will continue to be a fairly easy telescope object right through November, though at that point it starts to favor Southern Hemisphere observers. By November it’s down below Orion moving towards Canis Minor and so isn’t high enough to see until around midnight. Still, it should be fun to look at it November 4th at the same time the comet is on NASA’s “candid camera.” For a more detailed chart for finding it on other dates, go here.
Now why all the excitement about a comet? They have a long history of scaring people – of being signs of impending disaster – but of course that’s from the days when we knew much less about them than we do now. Observing a bright comet in 1957 is one of my first memories of the night sky. Later I got my start in free lance magazine writing when I introduced the readers of Popular Science to Comet Kohoutek – the “Comet of the Century.” Well it wasn’t. My writing career survived that since I was quoting reliable scientific authority, but it sure bruised my ego when Comet Kohoutek never developed to anything like what was predicted.
That was my most shocking comet until 2007. That’s when Comet Holmes suddenly burst on the scene – and I mean burst. One night it was an incredibly dim magnitude 17 object visible only to professional astronomers and amateurs with extremely large telescopes – and neither the professionals nor the amateurs were much interested in it. Such faint comets are fairly common, and this one had been seen on repeated visits since 1892. But in just a few nights it was easily seen with the naked eye. Amateur star gazers were dazzled. It was a bright, round cloud in the same general area of the sky as Hartley 2 will travel through this fall. Yet it was farther away – out beyond the orbit of Mars and oriented so that from Earth we were looking right down the barrel, so to speak. That is, it had a tail, but the tail of a comet always points away from the Sun and in this case it was pointing directly away from us.
That fuzzy blob just kept growing so that for a brief time it was actually the largest object in our solar system – though it was, of course, a big fluff ball consisting of next to nothing. And while scientists have their theories on how all this came about, no one is sure. The puzzles that continue to surround comets is why NASA, after visiting another periodic comet in 2005, decided to take a close look at this one as well. What will they see? Not much, I suspect. That is, not much in terms of what the average person will be able to interpret from the pictures. But presumably some more pieces of the comet puzzle will fall into place. As reported on Gary Kronk’s authoritative “Cometography” site – bookmark that one for future reference – here’s the plan:
The Deep Impact spacecraft, which studied periodic comet 9P/Tempel 1 in 2005, will examine 103P/Hartley 2 during the period of September 5 to November 25, 2010, as part of the EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation) mission. The closest approach occurs on November 4, when the minimum distance to the comet will be only 620 miles (1000 kilometers). The spacecraft will use two telescopes with digital color cameras and an infrared spectrometer. The latter instrument will determine the chemical composition of outbursts of gas from the comet’s nucleus.
And what will we be looking at? Comet Hartley is small even for a comet – about three-quarters of a mile across. (In comparison Comet Tempel 1 is thought to be 8.7 by 2.5 miles in size.) The revised general description for comets these days is a dirty snowball with a crust. Think of it a moment. Comets are not just any old chunk of rock. Millions are thought to reside in the Oort Cloud out at the farthest reaches of our solar system – about one light year away. They are thought to be pure examples of the original material of the cloud of gas and dust from which we were all formed – yes, I mean you and I. Seeing a comet up close is roughly akin to the ultimate of finding something really cool in your grandparents’ attic! So don’t get them confused with the other rocks that tend to float around nearby. We call those asteroids, and we worry some about them colliding with the Earth because such a collision would produce a major disaster. And, of course, a collision with a comet even as small as Hartley 2 would produce a major disaster. But comets and asteroids, while looking similar when far away from the Sun, are really quite different beasts. The visit to Comet Hartley 2 will include no pyrotechnics – Deep Impact has done its thing as far as throwing stuff at comets is concerned. Still, this picture of the mission to Tempel 1 when we shot a probe into it gives you a feel for what a comet looks like close up. It will be interesting to compare this photo with what we see in early November.
Meanwhile, out at Jupiter, the moons are doing their thing
An hour after sunset on October 1, Jupiter is already 15 degrees high in the eastern sky. It’s by far the brightest “star” in the east, so you can’t miss it. Each night it will a bit higher at the same time, and the higher it gets the easier it is to see detail and the moons using a telescope or binoculars. (Uranus is still nearby as well. If you want to try to spot it, see this post with charts from last month.)
Jupiter is a favorite sight in small telescopes – and to see the four Gallilean moons you really should use a telescope – any telescope. I have a tiny, 50mm “department store” telescope that mounted on an ordinary camera tripod does an excellent job showing the moons of Jupiter. It is possible to see them with binoculars if you have good eyesight and can hold the binoculars really steady – lean against a building, or better yet, mount them on a tripod.
Here’s a simulation of how the moons change position in a single evening. The changing tilt of the plane of the moons is due to the changing position of Jupiter in the night sky.
The moons shown at the start of the video are, from left to right, Callisto, Ganymede, Europa, and Io. The video simulation was made using Starry Nights Pro software.
They’re called the Galilean moons, of course, because Galileo discovered them four hundred years ago when he pointed a small telescope at the sky. The sight shook up the world! At that time most folks believed the Earth was at the center of the universe and everything revolved around it. A bit conceited, perhaps, but darn, watch the sky on any given night and it sure looks like we’re at the center of things! But here comes Professor Galileo with his new-fangled looking glass, and he points it at the bright “wandering star,” Jupiter, and what he sees is another world. Not only is it another world but a world with four little worlds clearly whirling around it! And of course he rushes right into print with this astounding discovery publishing his “Sidereus Nuncius” or “Starry Messenger” in March of 1610. Today many amateur astronomers enjoy studying details on this largest of planets. I love watching Jupiter’s four brightest moons. Here’s why:
- You can see them with any small telescope – even binoculars if you can hold them really steady.
- They do something! Most astronomical objects don’t change much over our lifetimes. Jupiter’s moons can go through significant changes in a single night as the video above shows.
- These four bright moons played a major role in changing our view of the universe.
- They even helped us determine the speed of light a couple of hundred years ago, something next to impossible to determine on Earth without modern, sophisticated instruments.
- For the telescope user they:
- duck in and out of Jupiter’s shadow (eclipse)
- hide behind the planet and suddenly pop out (occultation)
- cross in front of the planet providing a challenge for telescope users to spot them (transit – this is what Io does in the above video)
- and from time to time they cast their shadows on the giant planet – shadows visible in a backyard telescope as perfect round circles
- Hubble and modern spacecraft have shown us that Jupiter’s moons are full of surprises. No two are alike and all four are different than what scientists imagined before the spacecraft got out there and gave us an up-close and personal view.
- All of which is incredibly awesome when you understand that the little lights you see moving with grace, precision, and predictability are complete worlds in themselves the size of our moon or larger. (Ganymede is about 1.5x the diameter of our moon.)
How do you know which moon is which?
If you want to know which of Jupiter’s moons is where on any given night, use this neat little online utility provided by Sky and Telescope magazine. Now the basics. Jupiter has 63 moons, but only four of them are easily seen in small telescopes. Here are their names – in order moving outward from the planet – and links to more details about each.
Oh – and about determining the speed of light. Think about it. I believe Galileo once took a stab at this by stationing observers facing one another from different mountain peaks. They then uncovered a lantern at a predetermined time. No luck. Light is much too fast for this kind of experiment. Hey – light could go completely around the Earth more than seven times in a second! But here’s how Jupiter’s moons helped determine the speed of light more than 300 years ago! These kinds of discoveries always leave me in awe at how brilliant the discoverers were and how precisely they were able to make observations with tools that were not nearly as good as the inexpensive telescopes available to anyone today. The account which follows can be read in full here. It is from a posting by Michael Fowler of the University of Virginia Physics Department. One more thing to appreciate as you watch Jupiter’s moons.
The first real measurement of the speed of light came about half a century later, in 1676, by a Danish astronomer, Ole Römer, working at the Paris Observatory. He had made a systematic study of Io, one of the moons of Jupiter, which was eclipsed by Jupiter at regular intervals, as Io went around Jupiter in a circular orbit at a steady rate. Actually, Römer found, for several months the eclipses lagged more and more behind the expected time, but then they began to pick up again. In September 1676, he correctly predicted that an eclipse on November 9 would be 10 minutes behind schedule. This was indeed the case, to the surprise of his skeptical colleagues at the Royal Observatory in Paris. Two weeks later, he told them what was happening: as the Earth and Jupiter moved in their orbits, the distance between them varied. The light from Io (actually reflected sunlight, of course) took time to reach the Earth, and took the longest time when the earth was furthest away. When the Earth was furthest from Jupiter, there was an extra distance for light to travel equal to the diameter of the Earth’s orbit compared with the point of closest approach. The observed eclipses were furthest behind the predicted times when the earth was furthest from Jupiter. From his observations, Römer concluded that light took about twenty-two minutes to cross the Earth’s orbit. This was something of an overestimate, and a few years later Newton wrote in the Principia (Book I, section XIV): “For it is now certain from the phenomena of Jupiter’s satellites, confirmed by the observations of different astronomers, that light is propagated in succession (note: I think this means at finite speed) and requires about seven or eight minutes to travel from the sun to the earth.” This is essentially the correct value.
Less successful was an idea they had much later that they could solve the problem of finding one’s longitude – while at sea – by observing the moons of Jupiter. This was very seriously pursued because knowing longitude is critical to navigation and the accepted method involved using a very precise clock that kept correct time throughout a long sea voyage. It was hard enough to make a very precise clock – but one that retained its precision when subjected to the knocking about and unavoidable moisture that was part of any long voyage by sail? Nearly impossible. (See the wonderful book, “Longitude,” by Dava Sobel, for the story of how they did solve this.) But that said, as you watch the moons of Jupiter with a small telescope, try to imagine doing this on the heaving deck of a sailing ship with a telescope that is significantly cruder than the one you buy today! Then imagine that your life may depend on the result! Makes you appreciate the GPS if nothing else
So get out some night this month and take a close look at our neighbor, Jupiter. And if possible, see if you can see the moons. Newton is playing the tune, and the moons do the dance – music of the spheres indeed!
UPDATE – October 14, 2010 – Spent a nice hour this morning (4 am – 5 am) with Comet Hartley 2 – well, too much of that time trying to figure out how to find it with the Orion 110ED which is mounted on a parallelogram mount in my observatory and gets kind of complicated to point near the zenith. But I found the comet in just 30 seconds of looking with the 15X70 Celestron binos. That was easy and the comet seemed significantly larger and brighter than it was in my other observations of the past week. In ease of finding with those binoculars I would put it about halfway between M33 and M31. It seemed larger in the binoculars, as well. It was not nearly so easy inthe 8X50 finder – but certainly clearly visible.
Once I figured out how to twist and turn the 110 so it was pointed at the right section of sky – this is a very rich area about 6 degrees east of Mirfak – it showed very nicely in the 110mm. Yes, I could see a tail. I drew the stra field and sketched the comet and I had the orientation of the tail correct – but I was not confident I was seeing the tail until I checked my sketch against the charts. In other words, the tail is faint. In binoculars the comet reminds me very much of Holmes – especially since this is the same region Holmes inhabited when it was quite bright. But Hartley is moving much, much faster.
With the binos it was simply a blob – a puff of smoke among – and overlapping – some faint stars. In the 110mm it took on a solid nucleus – not star-like – larger – and a clearly defined coma. I kept changing the estimate of the coma’s size as I went from a 24mm Panoptic to Naglers of 13, 7, and 3.5 – that last is 236X. With each increase in magnification my estimates got smaller – not because I was seeing less of the comet – it took magnification well- but because I tend to think and object is taking up a greater percentage of the eyepiece than it is. Once more my sketch came to the rescue. It was easy to compare it to the known starfield and get an accurate measurement of 9 minutes of arc across for the clearly visible coma. I would put what I felt I could identify as a tail at about two or three times that. Be interesting to see what the photos show. My estimates are strictly about what I can see with what I’m using under dark and transparent skies -the transparency being a 4 out of 5 with 3 being average.
It is really a morning object now, with the moon starting to drown it out in the early evening.
UPDATE: October 7, 2010 -Comet Hartley 2 now visible in “normal” binoculars! Last night it was bright enough for me to see using 8X40 binoculars – significantly brighter than on October 2 when I last saw it. It was an easy target in 15X70 binoculars and I suspect that at this rate it will be reaching naked eye visibility – just barely – for those with really dark skies.. But even with small binoculars it’s still the kind of target that requires an observer to have some experience to find and see. For example, if you can find and see the large galaxy, M33, you certainly should be able to use the same instrument to see Hartley now.UPDATE: October 2, 2010 – While I would not discourage people from searching for Comet Hartley 2 right now, it is something that only experienced observers with dark skies will find easy to see at this moment. I have mag 5.5 skies and I could not see it last night while using 15X70 hand held binoculars when it was about 50 degrees above the horizon in the early evening. I did see it with an 80mm telescope and it was visible to one of three new observers who were visiting me last night when we used a 6-inch refractor. I would rate it a bit more difficult to see than M110, the fainter of the companion galaxies to M31.
Meanwhile, Spaceweather.com said early today that “some observers are reporting that comet 103P/Hartley 2 has reached the threshold of naked eye visibility.” I wrote them challenging those reports. No response, but I noticed that by 2 pm the earlier update had been taken down and a new one replaced it – one that makes no mention of “near naked eye visibility.
Sky and Telescope does report that it should brighten significantly over the coming week. I hope so. I worry about giving eager newcomers the wrong impression when we say – as I have done – that this is visible in binoculars. It may be in a few days – or to experienced observers with very dark skies right now – but not to more casual observers who have to cope with typical, suburban light pollution! While Comet Hartley 2 makes its closest approach on October 20 and should be brightest then, it will be difficult to see at that time because it will be fairly low in the evening sky and a nearly full moon will be drowning it out. I suspect the best time to see it will be around October 12 before the Moon offers significant interference. Perhaps then it will be visible to more casual observers using binoculars.
That said, it is an incredible sight in a large telescope. I observed it with my 15-inch reflector last night and was just blown away to think that so small an object - about three-quarters of a mile across – could kick out such an incredible dust cloud that we can see it from such a distance. If Comet Hartley 2 was a huge rock on the Moon I would not have a prayer of seeing it at just 3/4 of a mile across, even with the 15-inch. But it is about 50 times farther away. I haven’t done the calculations, but that cloud of dust I’m looking at has to be quite huge and is reflecting an awful lot of sunlight to make it visible to me. That’s awesome!